1. Field of the Invention
The invention relates to a laser drive circuit and a scanner drive circuit for a laser scanner control circuit which is used in an apparatus for scanning on a photo sensitive material by a laser beam and forming an image onto the photo sensitive material.
The invention relates to a monolithic driver IC in which the above laser scanner control circuit is formed on a single chip.
2. Related Background Art
In an image forming apparatus such as a laser printer or the like for scanning on a photo sensitive material by a laser beam and forming an image onto the photo sensitive material, a laser drive circuit to drive a laser and a polygon motor drive circuit are formed on different chips. As part of the laser drive circuit, a laser power control circuit controls laser power by logic or CPU control, or the like, and uses circuitry such as an A/D converter, a D/A converter, etc.
In a laser printer in which a print density can be switched, there is a case where a rotational speed of a polygon motor (scanner motor) must be changed and a laser beam power must be also changed in accordance with a set print density.
Therefore, a conventional laser scanner control circuit uses a two-signal construction in which a rotational speed control signal is received and the rotational speed of the scanner motor is changed and in which a laser beam power control signal is received and the laser beam power is changed, thereby to adjust the scanning speed of the laser and the laser beam power in accordance with the print density.
However, the rotational speed control signal and the laser beam power control signal are given independently so that there occurs a problem such that the number of input lines increases and input circuitry becomes complicated.
In addition, it is necessary to respectively supply the optimum signals according to the print density to a control circuit to generate the above two signals. It is difficult to optimize such signals unless the characteristics of a printer engine have been stored in a memory or the like.
Since both a digital logic section and an analog control section exist in the laser drive circuit, when the polygon motor drive circuit comprising a bipolar device and the laser drive circuit are formed on the same chip, it is necessary to construct the logic section by an IIL or to construct the logic section by a CMOS device and to construct the analog section by a bipolar device by using a BiCMOS process (process to form a circuit in which a bipolar device and a CMOS device mixedly exist).
When an IIL is used, therefore, there occurs a problem such that the laser power cannot be controlled at a high speed due to a frequency response of the IIL. When the BiCMOS process is used, there occur problems such that the manufacturing costs are high and the IC becomes expensive.
When the laser drive circuit and the polygon motor drive circuit are formed on the same chip, loss of each driver occurs on the same chip, so that there also occurs a problem such that a temperature of the chip rises.